Earliest Tertiary Break-up Volcanism in West Greenland: Mg-rich Volcanics from Nuussuaq and Disko

Lotte Melchior Larsen Geological Survey of Denmark and Greenland, Øster Voldgade 10,

DK-1350 Copenhagen K, Denmark


Asger Ken Pedersen Geological Museum, Øster Voldgade 5-7, DK-1350 Copenhagen K, Denmark

The Vaigat Formation on Nuussuaq and Disko is 1-2 km thick and consists dominantly of picrites with more than 15% MgO and olivine-phyric basalts with 10-15% MgO. The volcanic rocks rest unconformably on thick Cretaceous and minor Tertiary sediments of the West Greenland Basin. The known volcanism started in western Nuussuaq along a major, probably fault-controlled escarpment separating a shallow-marine continental platform in the east from deep sea in the west. The volcanic rocks quickly built up above sea level. With time, eruption centres moved eastwards across the
platform which was covered by eastwards-prograding hyaloclastite breccias and coeval subaerial lava flows.

The Vaigat Formation is divided into three major members, the Anaanaa, Naujánguit, and Ordlingassoq Members, and a number of minor intercalated, usually crustally contaminated members which serve as stratigraphical markers. The oldest rocks known are recovered from the oil exploration wells Marraat-1 and GANW#1 situated on the escarpment in western Nuussuaq, and these wells constitute the type section for the Anaanaa Member which has not previously been described. Volumetrically, the three major members constitute roughly 20%, 40% and 40% respectively of the uncontaminated rocks of the Vaigat Formation. The Anaanaa Member is less magnesian than the two later members, however picritic rocks are represented among the oldest drilled units. Average MgO contents for the three successive members are 12.7%, 16.3% and 16.4%, with very different frequency distributions. Only ca. 3% of the rocks have MgO<7.5%, and feldspar-phyric rocks are very rare. The liquids have undergone olivine fractionation and accumulation, and primary magmas with around 18% MgO are inferred.

The rocks are low-K tholeiites with low contents of incompatible elements and depleted geochemical and isotopic signatures. Some geochemical features vary systematically with stratigraphic height. The very oldest part of the succession (lower Anaanaa Member) has distinctly low levels of Ti, P, Sr, Zr and FeO* whereas the youngest part (Ordlingassoq Member) is significantly less depleted in these trace elements and has generally higher FeO* than the earlier rocks. (Sm/Yb)N ratios vary similarly with height, from 0.9-1.0 in the lower Anaanaa Member to 1.4-2.1 in the Ordlingassoq Member. La/Sm ratios vary irregularly and show no systematic evolution with height. Most rocks have (La/Sm)N<1, indicating a source component of depleted mantle.

The very depleted rocks low in the succession are similar to the depleted picrites on Baffin Island, and the less depleted rocks high in the succession are similar to the picrites on Svartenhuk Peninsula to the North. The differences between the areas are not place-dependent, rather dependent on time and/or tectonic conditions. The data can be explained from a combination of increasing depths of melting with time and mixing of varying amounts of melt from plume-type mantle and depleted mantle. The oldest rocks (lower Anaanaa Member) were produced by melting in spinel peridotite facies ((Sm/Yb)N<1) whereas all the other rocks show signs of melting also in garnet peridotite facies, most for the Ordlingassoq Member. All stratigraphic groups are dominated by rocks that are mixtures of melts from plume-type mantle and depleted mantle. In a Nb/Y vs Zr/Y diagram various stratigraphic groups form linear arrays that appear to converge towards a MORB melt composition with Zr/Nb around 60. "Enriched" rocks with REE patterns that do not require a depleted mantle component occur frequently in the Ordlingassoq Member but also sporadically at several lower levels. These may perhaps include a component of lithospheric mantle. The deepening with time of the melting levels may be due to the arrival of hotter mantle material that would start melting at deeper levels. Alternatively, there may be an increasing lid effect with time, the magmas erupting through successively thicker lithosphere as the eruption sites moved eastwards away from the continental margin.